chore(deps): upgrade to Rails 7.2.3.2, Ruby 3.4.10 and latest gems

Upgrades Rails from 7.1.6 to 7.2.3.2, Ruby from 3.4.6 to 3.4.10, and
brings the rest of the bundle up to date.

Two things were broken by Rails 7.2 and are fixed here:

* MigrationWaiter called `ActiveRecord::Base.connection.migration_context`,
  which moved to the connection pool in 7.2. This raised NoMethodError on
  boot for any deployment with `migration_waiter.enabled` set, and had no
  test covering it.
* `config.action_dispatch.show_exceptions = false` was deprecated in 7.1
  and removed in 7.2. Now `:none`.

`json` is pinned to < 3. Active Support 7.2 passes `quirks_mode:` to both
`JSON.generate` and `JSON.parse`, and json 3.0 removed that option, so an
unpinned json resolves to 3.x and makes every `to_json` raise - webhook
payloads included. Nothing else in the bundle constrains it. The pin can go
when we reach Rails 8.1, which no longer passes the option.

`dynamic_form` is dropped. It is unmaintained and from 1.3.1 requires Ruby
< 3.4, so it could never be updated again. Ten views used it, so
`error_messages_for` now lives in ErrorMessagesHelper and `f.error_messages`
in a form builder extension. The rendered markup is unchanged apart from a
heading and description the stylesheet had always hidden; the rules that
hid them are removed too.

`execjs` is unpinned. The `< 2.8` constraint dated from the Rails 6.0
upgrade and asset output is byte-identical without it.

Notable gem majors: puma 8, haml 7, secure_headers 7, sentry 7, jwt 3,
prometheus-client 5, highline 3, omniauth-rails_csrf_protection 2,
rspec-rails 8, shoulda-matchers 8, rubocop 1.91. secure_headers 7 changes
X-XSS-Protection from "1; mode=block" to "0", which is the correct modern
default and matches Rails' own.

`config.load_defaults` deliberately stays at 7.0. The pending 7.1 and 7.2
options are listed, commented out, in new_framework_defaults_7_1.rb and
_7_2.rb so they can be adopted in stages - several change the signed cookie
and cache formats and need a staged deploy. The vestigial
new_framework_defaults_7_0.rb is removed, as `load_defaults 7.0` already
applies all of it.

Adds tests for the areas this touched, which were mostly uncovered:
MigrationWaiter, the serialized columns that gate a defaults bump, JSON
encoding, HAML template compilation for all 103 templates, page rendering,
form error rendering and security headers. The assets Gemfile group now
includes :test so the view layer can be exercised at all.

spec/requests/rendering_spec.rb

@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+# These examples exist to exercise the view layer end to end - HAML, the asset
+# pipeline, the layout, form helpers and the session middleware. The rest of the
+# request specs only assert on redirects, so without these nothing in the suite
+# would notice if a Rails upgrade broke rendering.
+RSpec.describe "rendering", type: :request do
+  before do
+    host! Postal::Config.postal.web_hostname
+  end
+
+  describe "the login page" do
+    it "renders" do
+      get "/login"
+
+      expect(response).to have_http_status(:ok)
+      expect(response.media_type).to eq "text/html"
+      expect(response.body).to include "<form"
+      expect(response.body).to include "password"
+    end
+
+    it "renders the layout including compiled assets" do
+      get "/login"
+
+      expect(response.body).to include "<html"
+      expect(response.body).to match(/<link[^>]+stylesheets?/i).or match(/<link[^>]+\.css/i)
+      expect(response.body).to match(/<script[^>]+\.js/i)
+    end
+  end
+
+  describe "when not authenticated" do
+    it "redirects to the login page" do
+      get "/"
+
+      expect(response).to redirect_to("/login?return_to=%2F")
+    end
+  end
+
+  describe "when authenticated" do
+    let(:user) { create(:user) }
+
+    before do
+      post "/login", params: { email_address: user.email_address, password: "passw0rd" }
+    end
+
+    it "renders the organizations index" do
+      get "/"
+
+      expect(response).to have_http_status(:ok)
+      expect(response.body).to include "<html"
+    end
+
+    it "renders the user settings page" do
+      get "/settings"
+
+      expect(response).to have_http_status(:ok)
+      expect(response.body).to include user.email_address
+    end
+
+    # `f.error_messages` and `error_messages_for` used to come from the
+    # dynamic_form gem. The app provides them itself now (see
+    # ErrorMessagesHelper and config/initializers/form_builder_extensions.rb),
+    # so the rendered output is our responsibility to keep working.
+    describe "a form which fails validation" do
+      before do
+        patch "/settings", params: {
+          password: "passw0rd",
+          user: { first_name: "", last_name: "Smith", email_address: user.email_address }
+        }
+      end
+
+      it "re-renders the form" do
+        expect(response).to have_http_status(:ok)
+      end
+
+      it "renders the validation errors in the errorExplanation container" do
+        expect(response.body).to include %(<div id="errorExplanation" class="errorExplanation">)
+        expect(response.body).to include "<li>First name can&#39;t be blank</li>"
+      end
+    end
+
+    it "does not render an error container on a form which has no errors" do
+      get "/settings"
+
+      expect(response.body).to_not include "errorExplanation"
+    end
+  end
+end